Issue link: https://iconnect007.uberflip.com/i/1525368
46 PCB007 MAGAZINE I AUGUST 2024 temperature, take a small sample of the solu- tion and put it in a sealed bottle, then measure the conductivity and temperature of the rinse water for that process. When measuring the electroplating process, use the multimeter to measure the voltage drop from the rectifier to each anode bar. Later, a 50- or 100-milli-ohm shunt will be added into the series to measure the current. Do this every hour and pass the worksheet to the next shi if there is one. If the specific gravity (SG) varies through- out the day, then specific gravity control may be useful in controlling that process, especially if it corresponds to similar changes in the real- time analyzer. Specific gravity will change the most in processes that dissolve the photore- sists, add metals to the panel (like catalyst or electroless Cu), or remove copper from the panel (micro-etch or final etch).If you're con- sidering an upgrade to the UHDI processes, then fine-line etching will be a major issue. In his column, "Chemical and Equipment Con- trol of High-density Circuits," Don Ball of Chemcut said that the specific gravity controls for the final etch, be it cupric chloride or alka- line chloride, must be upgraded to control the specific gravity to 1.27 SG with a control limit of ±0.01 to 0.02 SG. is will cost $5,000 to $10,000. e specific gravity sensor that we devel- oped at Hewlett-Packard in 1975 (Figure 2) can meet these requirements and costs less than $50, although you have to make and calibrate it yourself. We had our maintenance depart- ment make them for us. Although the patent for the sensor has long since expired, it is still not noted in sensor or instrumentation books. At the time, this invention created quite a con- troversy at HP as the mechanical engineers told us, "It will never work." at was because they could not find any references to the prin- ciple in any sensor or measurement textbooks. "Sorry," we said, "but it does work. You forgot the finer points of the Archimedes Principle— for a submerged object, the buoyancy position is based on the displaced mass and that never changes, even if we move the calibration nuts up or down the threaded rod, which changes the gravity force applied. So, the only way the rod will float is if there is a change in the Figure 2: A very accurate and precise specific gravity sensor can be made by machining a solid-plastic rod, threading it, and then drilling out a specific depth to lower its weight. Two plastic nuts are machined of specific diameter to provide additional weight but applied at specific locations. When the required SG is achieved by the sensor, the magnetic reed switch will close and energize the circuit. continued on page 50